The present invention relates to an arc welding machine and a plasma cutting machine wherein an arc is generated between an electrode and a base metal in contactless fashion and so electric wave interference is reduced to drastically improve the arc start characteristic when an extension cable is used.
Conventionally, in an inconsumable electrode arc welding machine and a plasma cutting machine, high frequency voltage is applied across an electrode and a base metal only upon arc start to effect arc start in contactless fashion. A construction of this type of welding machine and plasma cutting machine will be described hereunder with reference to FIG. 4. In the Figure, when a start switch 6 is depressed, a contact 7 is closed because an arc generation detector 5 is not in operation, and a high frequency generator 9 is operated to feed power to a coupling unit 8. As a result, the coupling unit 8 superimposes high frequency power on power fed from a first power supply unit 1 across an electrode 4 and a base metal 3. Thus, a discharge is initiated in contactless fashion and then an arc is started by power fed from the first power supply unit. The contact 7 is opened by a signal from the arc generation detector 5 to stop the operation of the high frequency generator 9. In the case of a consumable electrode arc welding machine as shown in FIG. 5, on the other hand, when a start switch 6 is depressed, a first power supply unit delivers the output and a feed unit 10 of a consumable electrode (hereinafter called a wire) is operated, so that the wire touches the base metal to generate Joule heat by which the wire tip is fused off to ignite an arc, thus initiating welding.
In the conventional inconsumable electrode arc welding machine, arc start is effected by high frequency power and consequently, a high frequency voltage as shown in FIG. 11 is applied across the electrode and the base metal. At that time, radiation noise electric field intensity reaches up to about 70 dB.mu.V as shown in FIG. 12. Accordingly, electric wave interference was caused which interfered with neighboring televisions, radios or electronic apparatus, giving rise to breakdown of them in an extreme case, or the high frequency was so intensive that the electrode surface was roughened to deteriorate the arc start characteristic. Further, since the center frequency was a high frequency of about 1 MHz, the high frequency leaked on the way when a secondary side cable was extended and could not reach the tip of a torch, resulting in a failure to start an arc in the case of an extension cable of 50 m or more as shown in FIG. 13. Many problems as above have hitherto been encountered. In the case of the consumable electrode arc welding machine, on the other hand, an effort has been made to raise the current rise speed during start and as a result, the instantaneous arc start rate has been raised to about 90% but for promotion of FA, it is necessary to increase this rate to 100%. In the conventional techniques, the influence of slag (insulating material) adhered to the wire tip upon the start characteristic was difficult to avoid and it appeared that experimental data of FIG. 14 showed the limit of the contact start system (conventional technique).